This invention relates to a method of dyeing textile fibers using a vat acid dyeing method and, more particularly, to a vat acid dyeing method which utilizes additional reducing agent(s) to dye a variety of fibers to obtain deep shades and excellent washfastness.
Vat alkaline dyeing processes for use with textile fibers are known; see, for example, British Patent 534,085, International Patent Application WO96/04420, and U.S. Pat. No. 3,353,900. A vat neutral dyeing process has been disclosed for use with polypropylene fibers, American Dyestuff Reporter, March 1997, pp. 15-18, 66.
Vat acid dyeing processes have been disclosed in British Patents 709,150 (in which, however, the reduced dye is reoxidized before the dyeing step), 712,418 and 1,383,451, U.S. Pat. Nos. 2,627,449 and 3,527,556, Applications of Leuco Vat Acid Dispersion on Polyester (S. N. Chevli, Master""s Thesis, University of Leeds, UK, 1997), and American Dyestuff Reporter, Sep. 17, 1951, pp. 585-596.
The use of sulfinic acid reducing agents in alkaline vat dyeing has been disclosed in U.S. Pat. No. 6,007,587 and British Patent 1,430,179 and, in post-dyeing reduction clearing, in International Patent Application WO98/03725. The use of sodium formaldehyde sulfoxylate with vat dyes in printing and its instability in dilute acids have been disclosed in The Merck Index, Eighth Edition, Merck and Co., Inc., 1968, p. 959 and in Rongolit(copyright) C trade literature from BASF (TI/T 5952e, February 1997).
However, none of these processes provides adequate depth of shade, washfastness, or stain-resistance, and an improved dyeing method is still needed.
The process of the present invention for dyeing a fiber comprising a synthetic polymer selected from segmented polyurethanes, segmented polyurethaneureas, segmented polyetheresters, polyesters, polyamides, and poly(meta-phenylene isophthalamide), comprises the steps of:
(a) preparing a vat acid dye by:
(i) reducing a vat dye with a first reducing agent in water in presence of a surfactant at an alkaline pH; and
(ii) lowering the pH by the addition of a carboxylic acid;
(b) forming a dyebath by combining:
(i) said vat acid dye;
(ii) an aqueous solution of a carboxylic acid having a pH of about 5.2-6.5; and
(iii) a second reducing agent in an amount sufficient to maintain said dye in a reduced state, wherein said second reducing agent comprises at least about 20 mole %, based on the total of said second reducing agent, of a compound selected from the group consisting of xcex1-hydroxyalkyl-sulfinic acids having 1-6 carbon atoms, water soluble salts thereof, 1,2,4-trithiolane and mixtures thereof;
(c) contacting said fiber with said dyebath and heating to at least about 95xc2x0 C. for a time sufficient to dye the fiber; and
(d) oxidizing the dye in the fiber.
Also provided is a solid mixture comprising at least one vat acid dye, at least one carboxylic acid having 12-22 carbon atoms, at least one reducing agent selected from the group consisting of sodium dithionite, xcex1-hydroxyalkylsulfinic acids having 1-6 carbon atoms, water-soluble salts of such acids, sodium dithionite, 1,2,4-trithiolane, and mixtures thereof, and at least one surfactant.
It has now been unexpectedly found that deeply dyed textile fibers can be obtained by a vat acid dyeing process in which the pH is in a particular range during dyeing, additional selected reducing agent is added during the dyeing step, and the dyeing takes place at or above a specified minimum temperature. These dyed fibers have excellent washfastness and a low propensity to stain other fibers.
As used herein, xe2x80x9cspandexxe2x80x9d means a manufactured fiber in which the fiber-forming substance is a long chain synthetic polymer comprised of at least 85% by weight of a segmented polyurethane; fibers similarly comprising a segmented polyurethaneurea (a sub-class of segmented polyurethanes) are also within the definition of spandex. By xe2x80x9csegmentedxe2x80x9d is meant a polymer which has a plurality of segments of two classes. Each segment of the first class is the residue remaining after removal of the terminal hydrogen atoms from a polymeric glycol. The glycols typically have a melting point below 50xc2x0 C. and a number-average molecular weight above 600. Each segment of the second class contains at least one repeating unit of a fiber-forming polymer, typically having a melting point above 200xc2x0 C. xe2x80x9cVat dyexe2x80x9d means a colored aromatic compound containing two or more carbonyl groups conjugated with each other through double bonds. Vat dyes are generally used by reducing and dissolving them with a reducing agent in the presence of strong base, contacting the fibers to be dyed with the reduced dye, and then oxidizing the dye to its colored form in the fiber. Vat dyes are to be distinguished from xe2x80x9csolubilized vat dyesxe2x80x9d in that the latter are sulfuric acid esters of corresponding reduced vat dyes and have different chemical characteristics. xe2x80x9cVat acid dyeing processxe2x80x9d means a process in which, after reduction and dissolution of the dye, the dye solution is made acidic before contact is made with the fiber. xe2x80x9cBlendsxe2x80x9d of fibers means fibers which have been mingled with each other, for example by covering one fiber with another, by mechanically- or jet-mingling them, or by simultaneously knitting or weaving the fibers into a fabric.
The sodium salt of hydroxymethylsulfinic acid, generally available as the monosodium salt dihydrate, HOCH2SO2Na.2H2O, is also known as sodium formaldehyde sulfoxylate and hydroxymethanesulfinic acid (sodium salt); sodium dithionite is also known as sodium hydrosulfite.
In the process of the invention, a vat acid dye is formed by reducing a vat dye with a first reducing agent in water at an alkaline pH and lowering the pH of the resulting solution with a carboxylic acid, preferably to about pH 5.2-6.5 (more preferably to about 5.5-6.0), to form the vat acid dye in leuco form. The first reducing agent can be selected from sodium dithionite, 1,2,4-trithiolane, xcex1-hydroxyalkylsulfinic acids, water-soluble salts thereof, and mixtures thereof. The weight ratio of reducing agent to vat dye is preferably at least 2 to 1, more preferably at least 3 to 1, to fully convert the vat dye to its leuco form. A surfactant is added before lowering the pH of the solution of reduced alkaline vat dye. Anionic surfactants are preferred for improved stability against settling of the vat acid dye from the water. For greater storage stability, additional reducing agent can be added to the solution of vat acid dye, for example about 15 wt % based on total solution.
Examples of dyes that can be used in the method of the invention include Colour Index (C.I.) Vat Blue 1(indigo), C.I. Vat Violet 1 (indanthrene brilliant violet), C.I. Vat Green 1 (indanthrene brilliant green), C.I. Vat Orange 15 (duranthrene orange), C.I. Vat Red 41 (thioindigo), C.I. Vat Red 13 (indanthrene red), and mixtures thereof, but any vat dye or vat dye mixture that is stable to the conditions of the dyeing process and can subsequently be oxidized to its corresponding pigment form can be used. For instance, a deep black color can be obtained on polyester bicomponent fibers (for example, comprising poly(ethylene terephthalate) and poly(trimethylene terephthalate)) using a mixture of 3 wt % Vat Blue 1, 6 wt % Vat Blue 18 and 1.5 wt % Vat Orange 15. Dyes having one or two carbonyl groups (for example dibenzanthrones) were observed to give deeper shades than those having nitrogen-containing rings, and the former are preferred. Further, the fiber(s) to be dyed can affect the selection of the dye. For example, C.I. Vat Red 13 was observed to give deeper shades on polyamides, polyurethanes, and cotton than on polyester.
The vat acid dye and a second reducing agent which contains at least about 20 mole % (based on total second reducing agent) of a compound selected from the group consisting of 1,2,4-trithiolane, xcex1-hydroxyalkylsulfinic acids having 1-6 carbon atoms, water-soluble salts of such acids (for example sodium, zinc, or potassium salts), and mixtures thereof are combined with an acid solution. Examples of useful sulfinic acid salts include HOCH2SO2M (metal salt of hydroxymethylsulfinic acid; xe2x80x9cMxe2x80x9d represents a metal cation), HOCH(CH3)SO2M, HOCH(CH3)SO2M, HOCH(C2H5)SO2M, HOCH(C3H7)SO2M, HOC(CH3)2SO2M, HOC(C2H5)2SO2M, the homologous series of HOC(CH3)(C2H5)SO2M to HOC(CH3)(C4H9)SO2M, HOC(C2H5)(C3H7)SO2M, HOC6H10SO2M, and the like. Sodium hydroxymethylsulfinate is preferred due to its commercial availability and high solubility in water.
Fibers which can be dyed by the process of the invention include those comprising synthetic polymers selected from segmented polymers including segmented polyurethanes, segmented polyurethaneureas, and segmented polyetheresters; polyesters including poly(trimethylene terephthalate), poly(tetramethylene terephthalate), and poly(ethylene terephthalate); polyamides including poly(hexamethylene adipamide) and polycaprolactam and poly(meta-phenylene isophthalamide. Copolymers related to such polymers by the inclusion of comonomers can also be dyed by the present process. Bicomponent fibers dyeable by the process of the present invention include poly(trimethylene terephthalate)//poly(ethylene terephthalate), in which either polymer can be a copolyester for example with isophthalate, and poly(hexamethylene adipamide)//poly(hexamethylene-co-2-methylpentamethyl-ene adipamide), in which the copolyamide component is about 20-40 mole % 2-methylpentamethylene adipamide units.
When the fiber comprises a synthetic polymer selected from poly(tetramethylene terephthalate), poly(ethylene terephthalate), poly(hexamethylene adipamide), poly(metaphenylene isophthalamide), and polycaprolactam, the dyebath can have a pH of about 5.2-6.5, preferably about 5.5-6.0. When the fiber comprises a synthetic polymer selected from segmented polyurethanes, segmented polyurethaneureas, segmented polyetheresters, and poly(trimethylene terephthalate), the dyebath can have a pH of about 4.0-6.9, preferably about 5.2-6.5, so that the vat dye remains in its acid form. Blends of such fibers can also be dyed by the process of the invention.
It was particularly surprising that water-soluble salts of xcex1-hydroxyalkylsulfinic acids had a beneficial effect in a vat acid dyeing process, since such compounds are said to be readily decomposed by dilute acid. Less than about 20 mole % of such a second reducing agent(s) confers little advantage in washfastness and only slightly improved depth of shade. The amount is preferably less than about 85 mole % of the total second reducing agent(s) in the dyeing step. With increasing levels of such acid salt reducing agent, the depth of shade, though still acceptable and useful, declines somewhat, and above about 85 mole %, most of the increase in the tensile strength of the dyed fabrics has been achieved. The practitioner can adjust the relative amounts of reducing agents used in the dyeing step within the scope of the invention to achieve the desired balance of depth of shade and fabric tensile strength.
The carboxylic acids found to be useful in forming the vat acid dye and in maintaining an acid pH during the dyeing step can include acetic acid, formic acid, citric acid, lactic acid, and mixtures thereof. Citric acid and formic acid are preferred for better depth of shade and washfastness.
During the dyeing step, the reducing agent and the acid are used in amounts sufficient to maintain the vat acid dye in a reduced state. The total amount of reducing agent(s) and acid(s) used can depend on the dyeing apparatus used. An apparatus which permits greater exposure of the dyebath to air and the space surrounding the dyebath will require more reducing agent and acid than an apparatus that restricts air oxidation and acid evaporation.
Optionally and for greater ease of handling, a pellet or cake can be made by contacting the alkaline leuco form of the dye (for example the disodium salt) with a carboxylic acid having 12-22 carbon atoms, such as stearic acid. The resulting solid mixture can comprise the vat dye in acid form even when the pH of the mixture is as high as 7. The vat acid dye is surprisingly stable against air oxidation when thus mixed with the carboxylic acid. For even greater stability, a reducing agent can be included in the solid mixture. The solid mixture can be used with the dyeing process of the present invention, with vat acid dyeing processes outside the scope of the present invention, and with conventional (alkaline or neutral) vat dyeing processes. In the last instance, the basic conditions of the dyebath are sufficient to re-convert the dye to its alkaline leuco form. Solid formulations of reducing agents such as sodium dithionite have been disclosed in U.S. Pat. No. 6,007,587 and British Patent 1,415,837.
The fiber, for example in a fabric, as a skein, or on a wound package, is then contacted with the dyebath, and the dyebath is heated to at least about 95xc2x0 C., preferably at a rate in the range of about 0.5-2.0xc2x0 C. per minute for deep dye shades. It is more preferred that the heating rate be in the range of about 0.5-1.0xc2x0 C. per minute for even deeper shades. Fabric tensile properties can decline somewhat with slower heating rates, which can be adjusted to achieve the desired balance depth of shade and fabric tensile strength.
When the synthetic fiber comprises a polymer selected from polyurethanes, polyurethaneureas, poly(hexamethylene adipamide), poly(trimethylene terephthalate), and poly(tetramethylene terephthalate), the dyebath can be heated to at least about 100xc2x0 C., and when the fiber comprises poly(ethylene terephthalate), the dyebath can be heated to at least about 115xc2x0 C. With bicomponent fibers comprising poly(trimethylene terephthalate) and poly(ethylene terephthalate), it was observed that when the dyeing temperature was increased from 115xc2x0 C. to 130xc2x0 C., the depth of shade and washfastness increased.
After a time sufficient for the dye to penetrate the fiber, the dye in the fiber is oxidized to its pigment form with an oxidizing agent. In the dye oxidation step, oxidants such as hydrogen peroxide, Oxydant Diresul(copyright) BRI and Oxydant Diresul(copyright) SZ (both based on sodium bromate, from Clariant) can be used. Hydrogen peroxide is preferred, because among the oxidants tested, it resulted in the best dye depth of shade and dye washfastness.
Finally, steps such as reduction clearing and/or soaping can be carried out if loosely fixed dye remains on the fiber surface.
Optionally, the oxidation step can be carried out after the dyeing step by adding sufficient oxidant to the dyebath to oxidize both the dye in the fiber and any residual reducing agent in the dyebath, without draining, refilling, and re-heating the dyebath. Doing so can save energy and water.
As an additional alternative, a reduction clearing step can be carried out before the oxidation step by cooling the dyebath (containing reducing agent and acid) to no higher than about 95xc2x0 C., adding enough base to bring the pH to at least about 10 and carrying out the oxidation step. This can save reducing agent as well as energy and water.
Blends of synthetic fibers with natural fibers, for example cotton, lyocell, and the like, can also be dyed by the process of the invention, with the modification that a conventional vat alkaline dyeing step is carried out after the vat acid step and before the oxidation step to dye the cellulosic fibers in the blend. For example, after the vat acid dyeing step described above for the synthetic fibers, the dyebath can be cooled to no higher than 95xc2x0 C. (preferably no higher than 50xc2x0 C.), and sodium hydroxide (sufficient to raise the pH to at least about 10), alkaline reduced (leuco) vat dye (for example 2-3 wt % based on fiber), optionally more sodium dithionite (for example 1.5 wt % based on dyebath), optionally sodium sulfate (for example 2 wt % based on dyebath) and optionally nonionic detergent (for example 4 g/l of dyebath) can be added. The dyebath temperature can then be adjusted to at least about 60xc2x0 C., maintained for 45 minutes, and cooled. Such an additional step also accomplishes reduction clearing on the synthetic fibers, as described above. Finally, the dye in the fiber can be oxidized and the fibers optionally soaped, as described elsewhere herein.
A buffer solution can be used to maintain a constant pH during any step of the inventive process. Such a solution can be prepared from a mixture of 0.2 M of the carboxylic acid and 0.1 M sodium dihydrogen phosphate, in the appropriate proportions.
The polyester bicomponent yarn used in the Examples was a 70 denier (78 decitex), 34-filament yarn comprising poly(trimethylene terephthalate) and poly(ethylene terephthalate), made by the following method. Poly(trimethylene terephthalate) (60 wt %, 1.24 intrinsic viscosity, xe2x80x9cIVxe2x80x9d) and poly(ethylene terephthalate) (40 wt %, Crystar(copyright) 4415, a registered trademark of E. I. du Pont de Nemours and Company, 0.51 IV) were melted in independent melt systems, transported to a spinneret, and spun side-by-side into a cross-flow quench. Each component contained 0.3 wt % TiO2. An organic ester-based emulsion finish was applied (5 wt %) to the yarn. The yarn was passed around a feedroll, through a 170-xc2x0 C. steam draw jet, and then around a draw roll at a draw ratio of 2.9. The yarn was then passed through a 180-xc2x0 C. hot chest containing two rolls at a second draw ratio of 1.3. About 7.5 turns were taken around the hot chest rolls. The yarn was passed around a puller roll and a letdown roll and then wound onto a paper core tube. The resulting fibers had a tenacity of 3.5 g/d (3.1 dN/tex), elongation-to-break of 13%, and a crimp contraction value of about 55%. Crimp contraction levels in the polyester bicomponent fiber used in the Examples were measured by hanging a loop of fiber from a holder with a 1.5 mg/denier (1.35 mg/dtex) weight attached to the bottom of the loop and measuring the length of the loop. Then, a 100-mg/den (90 mg/dtex) weight was attached to the bottom of the loop, and the length of the loop was measured again. Crimp contraction was calculated as the difference between the two lengths, divided by the length measured with the 90-mg/dtex weight.
K/S, which indicates depth of shade at a chosen wavelength, can be obtained from the Kubelka-Munk equation       K    /    S    =                    (                  1          -          R                )            2              2      ⁢      R      
wherein K is the absorption coefficient, S is the scattering coefficient, and R is the reflectance (the ratio of reflected to incident light). When K/S is plotted against wavelength over a range of 300-700 nm, f(k) is the area under the curve. To obtain the colorimetric data reported in the Examples, f(k) values were measured with an X-Rite, Inc. (Match-Rite model) reflectance spectrophotometer (Grandville, Mich.) using X-Rite personal computer software. A D65 light source was used; the specular component of the light was excluded, and the ultraviolet component was included. A 10xc2x0 observer angle was used. For each test, the fabric sample was folded once so that a double thickness was presented to the light. Four readings were taken on each sample, the sample having been rotated 90xc2x0 from each previous reading in order to avoid orientation effects. A higher f(k) value indicates better depth of shade in the dyed fabric.
The washfastness test method used in the Examples was ISO CO6/C2, and spectrophotometric measurements were taken on dyed fabric samples before and after they were so tested. Changes in fabric color resulting from the five washings stipulated in the test method are reported in the Examples as a percent change in f(k) and as xcex94E (calculated according to the CMC (I,c) equation, as described in xe2x80x9cColour Physics for Industryxe2x80x9d, Second Edition, Roderick McDonald, ed., Society of Dyers and Colourists, pp 151-155, 1997):       Δ    ⁢          xe2x80x83        ⁢          E              CMC        ⁢                  (                      l            :            c                    )                      =            [                                    (                                          Δ                ⁢                                  xe2x80x83                                ⁢                                  L                  *                                                            IS                L                                      )                    2                +                              (                                          Δ                ⁢                                  xe2x80x83                                ⁢                                  C                  ab                  *                                            cSc                        )                    2                +                              (                                          Δ                ⁢                                  xe2x80x83                                ⁢                                  H                  ab                  *                                                            S                H                                      )                    2                    ]              1      /      2      
wherein xcex94L*, xcex94C*ab and xcex94H*ab are, respectively, the CIELAB lightness, chroma, and hue differences between the unwashed and washed samples, I and c are the tolerances applied, respectively, to differences in lightness and chroma relative to hue differences (the numerical values used in a given situation being substituted for the characters I and c, for example CMC(2:1), whenever there might be ambiguity), and:       S    L    =                              0.040975          ⁢                      L            s            *                                    1          +                      0.01765            ⁢                          xe2x80x83                        ⁢                          L              s              *                                          ⁢              xe2x80x83            ⁢              if             ⁢              xe2x80x83            ⁢              L        s        *              ≥    16  
and SL=0.511 if L*S less than 16;       S    C    =      0.638    +                  0.0638        ⁢                  xe2x80x83                ⁢                  C                      ab            ,            S                    *                            1        +                  0.0131          ⁢                      xe2x80x83                    ⁢                      C                          ab              ,              S                        *                              
and SH=SC(TF+1xe2x88x92F)             wherein        ⁢          xe2x80x83        ⁢    F    =            [                                    (                          C                              ab                ,                S                            *                        )                    4                                                    (                              C                                  ab                  ,                  S                                *                            )                        4                    +          1900                    ]              1      /      2      
and
T=k1+[k2 cos(hab,S+k3)]
wherein L*S, C*ab,S, and hab,S are respectively the CIELAB lightness, chroma, and hue angle (in degrees) of the unwashed sample, and
k1=0.36, k2=0.4, k3=35 if Hsxe2x89xa6164 or Hsxe2x89xa7345,
and
k1=0.56, k2=0.2, k3=168 if 164 less than Hs less than 345.
Small changes in f(k) and low values of xcex94E indicate good washfastness. In qualitative tests, each fabric was rated on a 1-5 scale (1 poor, 5 excellent) after the five washings. For fibers dyed according to the process of the invention, all qualitative washfastness test results ratings were at least xe2x80x9c4-5xe2x80x9d, and most were xe2x80x9c5xe2x80x9d. Qualitative washfastness evaluations were also made of the staining propensity using the same test method on fabrics dyed by the process of the present invention. In this case, strips of the test fabric were washed adjacent to other fabrics of fibers such as wool, acrylic, poly(ethylene terephthalate), nylon 6-6, and cotton and cellulose acetate. The same 1-5 scale was used, and staining ratings for all the fabrics dyed by the process of the invention were at least xe2x80x9c4xe2x80x9d (very good), and most were xe2x80x9c5xe2x80x9d.
The rubfastness test method used was ISOxc3x9712/1. Tensile tests made on the dyed fabrics were conducted according to British Standard 2576:1986.
Unless otherwise noted, all chemicals were reagent grade obtained from Aldrich Chemical Company, and all scouring, dyeing, reduction clearing, and soaping steps were carried out in sealed stainless steel dyepots of 300 cm3 capacity, housed in a Roaches xe2x80x9cPyrotec Sxe2x80x9d laboratory dyeing machine, the carousel of which was operated at 55 rpm. In each step, the solutions were added to the dyepot and warmed to 40xc2x0 C. before adding the fabric, each sample of which weighed 10 g. The fabric was pre-wet with distilled water before the dyeing step. Between each step the dyepots were emptied and cleaned by rinsing them with warm tap water and then with distilled water, after which they were dried. Unless otherwise noted, 20 ml of aqueous solution was used for each gram of fabric in dyeing and reduction clearing, and 25 ml of aqueous solution was used per gram of fabric in scouring. After each dyeing and finishing step, the fabric was air-dried by hanging it up overnight.